CN113710355A

CN113710355A - Non-invasive mixing of liquids

Info

Publication number: CN113710355A
Application number: CN202080029614.6A
Authority: CN
Inventors: 保罗·路易斯·玛莉亚·约瑟夫·万妮尔; 拉贾特·巴德瓦伊; 赫特-扬·阿徳里安·范赫鲁拿斯泰纳; 厄尔·劳伦斯·文森特·格特希尔; 科内利斯·皮特勒斯·马库斯·鲁兰斯
Original assignee: Netherlands Organization For Applied Scientific Research
Current assignee: Netherlands Organization For Applied Scientific Research; Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 2019-03-14
Filing date: 2020-03-12
Publication date: 2021-11-26
Also published as: WO2020185085A1; EP3938090A1; EP3708247A1; JP2022525598A; US20220143563A1

Abstract

An apparatus (100) for mixing a fluid (F) comprises a mixing vessel (10) having a vessel wall (11) for containing the liquid (F). One or more acoustic transducers (21, 22) are arranged on the vessel wall (11) and are configured to generate respective sound waves (W1, W2) which are directed into the fluid (F) for inducing respective flow patterns (F1, F2) in the fluid (F) by the acoustic flow. A controller (15) configured to control the acoustic transducers (21, 22) to automatically switch between the generation of different sound waves (W1, W2) to cause switching between different flow patterns (F1, F2).

Description

Non-invasive mixing of liquids

Technical Field

The present disclosure relates to the mixing of fluids, such as liquids, such as milk.

Background

In many industries, for example, the food, chemical and pharmaceutical industries, dispersions, suspensions and emulsions need to be mixed or kept mixed. Generally, there are powerful drivers for hygiene and sterility to maximize the shelf life of the product. In these industries, conventional mixing involves inserting a component (e.g., an impeller) into the dispersion, suspension, or emulsion to facilitate mixing. However, the part may need to be cleaned, which may take time. Furthermore, each time a wash is performed, the wash may involve labor and energy costs. Thus, in the food (such as dairy) industry, for example, cleaning is a significant part of the cost.

Ultrasonic cleaning baths use ultrasonic waves to clean/mix/augment chemical reactions. However, these are usually based on high power ultrasound-with high intensity only at a small defined point-where the working principle is mainly dominated by cavitation and locally induced temperature increase. The application of ultrasonic mixing/sorting may also occur in microfluidic devices. Standing waves are often used, which is relatively easy to achieve in microfluidic devices, but not feasible for larger devices. Unfortunately, mixing based on known ultrasound may not be suitable for liquids that are easily damaged, such as dispersions and emulsions. For many liquids, there is an allowable upper limit to the peak liquid velocity or induced shear stress. For example, for milk, the upper limit may be determined by breaking down the protein-fat structure under high shear stress. However, keeping below the upper limit may result in insufficient mixing.

There is therefore a need for further improved mixing of fluids, which may alleviate the disadvantages of the known method, while maintaining at least some of the advantages of the known method.

Disclosure of Invention

Aspects of the present disclosure relate to apparatuses and methods for mixing fluids (e.g., liquids). The mixing vessel has a vessel wall, and the mixing vessel can be used to hold a fluid. One or more acoustic transducers may be arranged on the wall of the container. The acoustic transducer may be configured to generate a corresponding acoustic wave that is directed into the fluid. This can cause a corresponding flow pattern (acoustic flow) in the fluid. For example, the flow pattern may be described by the respective flow direction and/or flow velocity of the fluid at one or more locations in the mixing vessel. Typically, mixing is achieved by carrying a flow of fluid and/or particles throughout the vessel.

Preferably, the one or more acoustic transducers are controlled to switch automatically between the generation of different sound waves. This may cause switching between different flow patterns to improve fluid mixing without having to increase the driving power. The inventors have realized that without switching the acoustic wave generation, a fixed or steady state flow pattern may be formed, for example, in which the flow direction and velocity at various locations in the fluid no longer change. For example, in a fixed flow pattern, laminar flow may be formed, with minimal mixing occurring in the laminar flow. Furthermore, the fixed flow pattern may include areas where the fluid remains stagnant. For example, different flow patterns can be formed by switching the flow direction and/or flow velocity at one or more locations, preferably throughout the vessel. Advantageously, switching between different flows may disrupt laminar flow and/or stagnant regions in the vessel, e.g., creating vortices that may improve mixing performance. Thus, mixing efficiency can be improved by switching different mixing modes without damaging the fluid, rather than, for example, increasing the power of the transducer (which may damage the fluid due to excessive flow/shear).

Drawings

These and other features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIGS. 1A and 1B show a circular flow pattern;

FIGS. 2A and 2B show the flow pattern of the helix;

FIGS. 3A and 3B illustrate flow patterns having opposite flow directions;

FIGS. 4A and 4B illustrate a mixing vessel in the shape of a ring or torus;

FIGS. 5A and 5B illustrate sound waves oriented at an angle relative to opposing walls;

FIGS. 6A and 6B illustrate the combination of acoustic streaming and radiation force;

FIG. 7A shows pressure distribution intensity;

fig. 7B shows interference between sound waves.

Detailed Description

The terminology used to describe particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and/or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and/or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It should be further understood that when a particular step of a method is referred to as being subsequent to another step, unless otherwise indicated, the particular step of the method can be directly subsequent to the other step or one or more intermediate steps can be performed before the particular step is performed. Also, it should be understood that when a connection between structures or components is described, the connection may be established directly or may be established through intermediate structures or intermediate components, unless otherwise specified.

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-sectional illustrations of potentially idealized embodiments and intermediate structures of the invention. Like numbers refer to like elements throughout the specification and drawings. Related terms and derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing figures. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless otherwise stated.

Fig. 1A and 1B illustrate aspects of an apparatus 100 embodied for mixing a fluid F described herein. In general, the apparatus 100 comprises a mixing vessel 10 having a vessel wall 11 for containing a fluid F. As described herein, the apparatus typically has at least one transducer disposed on the vessel wall 11 for mixing the fluid F. In a preferred embodiment (e.g., as shown), a plurality of

acoustic transducers

21, 22 are arranged on the vessel wall 11. In another or further embodiment, the one or more

acoustic transducers

21, 22 are configured to generate respective acoustic waves W1, W2, which are directed into the fluid F for inducing (preferably by acoustic flow) respective flow patterns F1, F2 in the fluid F. Aspects described herein may also be embodied as methods for mixing a fluid F. Generally, the method includes containing a fluid F in a mixing vessel 10 and generating respective acoustic waves W1, W2 directed into the fluid F for inducing respective flow patterns F1, F2 in the fluid F by the acoustic flow.

In some embodiments, the controller 15 is configured to control one or more

acoustic transducers

21, 22. In a preferred embodiment, the controller is configured (e.g., programmed) to automatically switch between the generation of different sound waves W1, W2. This may cause switching between different flow patterns F1, F2 to improve fluid mixing. Similarly, the method may further comprise switching (by a controller or otherwise) between the generation of different sound waves W1, W2 to cause switching between different flow patterns F1, F2. While switching between different flow patterns may provide synergistic advantages in combination with the different aspects described herein, it is also contemplated that at least some of the teachings herein may be applied without switching. In particular, the aspects and advantages described herein (e.g., circular/helical flow patterns, opposing/shear flow patterns, flow patterns angled to opposing walls, and/or flow patterns that impinge acoustic waves on a liquid/gas interface), vessel/transducer configurations, and operating parameters may also be applied without switching to achieve at least some of the advantages of efficient fluid mixing.

In some embodiments, actuation (with or without intermittent switching) is maintained for a relatively long period of time, e.g., longer than one minute, ten minutes, half an hour, or more. For example, some fluids (e.g., milk) may need to be mixed continuously to maintain desired characteristics. Thus, mixing can be maintained as long as the fluid is stored in the mixing vessel 10. In some embodiments, for example, when mixing is deemed sufficient, actuation may be turned off. For example, the actuation may also be temporarily turned off in cycling between different actuation modes.

In some embodiments, the

acoustic transducers

21, 22 are configured to induce a first flow pattern F1 by generating a first set of sound waves W1 over a first time period T1, and then automatically switch to induce a second, different flow pattern F2 by generating a second, different set of sound waves W2 over a second time period T2. In one embodiment, the time periods T1, T2 may be selected to correspond to the time required for a fixed flow pattern (e.g., predominantly laminar flow) to develop in the vessel. By switching the transducer around (or before) this time, the fixed flow pattern can be broken to maintain optimal mixing conditions. For example, each time period T1, T2 may be at least one second, two seconds, five seconds, or ten seconds or more. For example, each flow pattern may be maintained between one second and one hundred seconds, preferably between five seconds and thirty seconds, or between ten seconds and twenty seconds, before switching to the next flow pattern.

In some embodiments, a first subset of transducers 21 is configured to induce a first flow pattern F1, while a second, different subset of transducers 22 is configured to induce a second flow pattern F2. In one embodiment, the respective subsets of transducers may be exclusive. For example, transducers belonging to the first subset do not belong to the second subset, whereas transducers belonging to the second subset do not belong to the first subset. As shown, advantageously, each subset of

transducers

21, 22 may be specifically arranged to induce a particular respective flow pattern F1, F2. In other or further embodiments, one or more transducers may be shared between subsets (not shown here). For example, some transducers belonging to the first subset may also belong to the second subset, while other transducers may be unique to the respective subset.

In some embodiments, different flow patterns F1, F2 are created by (the controller 15) switching actuation between different subsets of

actuators

21, 22. For example, to create the first flow pattern F1, the first set of actuators 21 is actuated. For example, to create the second flow pattern F2, a different second set of actuators 22 is actuated. In one embodiment, the different flow patterns F1, F2 may be generated by switching operating parameters of one or more actuators belonging to one or more groups. For example, the switching of the flow pattern may be altered by switching one or more actuators from a first actuation frequency to a second, different actuation frequency.

In some embodiments, for example, the flow pattern is abruptly changed by switching actuation from one mode of operation to a completely different mode within one second. For example, the second group 22 of one or more acoustic transducers is turned on at the same time or shortly after the first group 21 of one or more acoustic transducers is turned off. The abrupt switching may, for example, induce the formation of a vortex by an abrupt change in flow direction to improve mixing. In other or further embodiments, the flow pattern may be switched to gradually change the flow. For example, the actuation of the first group 21 of one or more acoustic transducers is ramped down, while the actuation of the second group 22 of one or more acoustic transducers is ramped up (e.g., over a time period of one or more seconds, e.g., up to ten seconds or more).

In some embodiments, the one or more

acoustic transducers

21, 22 are configured to alternate between two, three, four, five or more different flow patterns. The greater the number of different flow patterns, the better they can complement in terms of efficient mixing of the fluids. Preferably, the flow patterns are as different as possible, e.g. having completely different flow directions.

In some embodiments, the first flow pattern F1 has a first flow direction V1 and the second flow pattern F2 has a different second flow direction V2. Advantageously, switching between flow patterns F1, F2 having different flow directions V1, V2 may disrupt laminar flow and/or counteract stagnant regions in the mixing vessel 10. In one embodiment, the flow direction V1 is generally opposite the second flow direction V2. For example, the average flow direction V1 of the first flow pattern F1 at a location in the mixing vessel 10 may be at a relatively large angle, e.g., at an angle of greater than ninety degrees, greater than one hundred twenty degrees, greater than one hundred fifty degrees, up to one hundred eighty degrees (diametrically opposed), relative to the average flow direction V2 of the first flow pattern F1 at the same location. For example, the first flow pattern F1 may be clockwise and the second flow pattern may be counter-clockwise. In another or further embodiment (not shown here), the first flow direction V1 is generally transverse to the second flow direction V2, e.g., wherein the included angle between the average flow directions V1, V2 is between forty-five degrees and one hundred thirty-five degrees.

In some embodiments, different flow directions may be achieved by sound waves W1, W2 originating from different

acoustic transducers

21, 22 and/or using waves/transducers oriented at different angles α 1, α 2. Generally, the sound waves W1, W2 are directed into the fluid F along respective acoustic axes a1, a 2. In one embodiment, for example, as shown, the acoustic axes a1, a2 are at respective angles α 1, α 2 relative to a normal An of the (inner) vessel wall 11 for tangential main flow components of the respective fluid flows F1, F2 to the vessel wall 11. For example, the angle α is greater than ten degrees (plane angle), preferably greater than twenty degrees, or even greater than thirty degrees, greater than forty degrees, or greater than fifty degrees, for example, between forty and eighty degrees. The greater the angle a (up to ninety degrees), the more likely the fluid will begin to begin a flow pattern directed along the wall.

In some embodiments, for example, as shown, the first acoustic transducer 21 has An acoustic axis a1 with An acoustic axis a1 at a first angle α 1 relative to the normal An of the vessel wall 11, and the second acoustic transducer 22 has An acoustic axis a2 with An acoustic axis a2 at a second angle α 2 relative to the normal An of the vessel wall 11. In one embodiment, the angles α 1, α 2 may be the same, but oriented in different directions, for example. For example, as shown, the angles α 1, α 2 may be oppositely oriented along the circumference of the container wall 11. Alternatively, or additionally, the directions of the angles α 1, α 2 relative to the respective normal An may also have transversely oriented components (not visible here).

In some embodiments, the one or more

acoustic transducers

21, 22 are arranged outside the mixing vessel 10, i.e. on the opposite side of the vessel wall 11 with respect to the fluid F. For example, it may be advantageous to keep the transducer on the outside in terms of maintenance and/or keeping fluid out of contact. In other or further embodiments, one or more

acoustic transducers

21, 22 may be partially or completely buried in the vessel wall 11 in order to more easily couple waves into the fluid. Preferably, the one or more

acoustic transducers

21, 22 are not in contact with the fluid, e.g. to prevent contamination.

In some embodiments, the wedge element 11w is arranged between the

acoustic transducers

21, 22 and the vessel wall 11 to determine the angle α. In another or further embodiment, the vessel wall 11 itself may contain or form a wedge-shaped surface against which one or more

acoustic transducers

21, 22 may be mounted. One or more

acoustic transducers

21, 22 may also be partly buried within the container wall 11, e.g. at an angle with respect to the (inner) surface normal or in another way. Although in the illustrated embodiment one or more

acoustic transducers

21, 22 are mounted at an angle on the wedge element, the transducers may alternatively be mounted in the same direction (in-plane) as the wall, for example by mounting a complementary second wedge element (not shown) on the first wedge element. For example, the interconnected wedge-shaped elements may have different acoustic impedances for refracting the acoustic wave at a desired angle.

In some embodiments, the one or more transducers are configured to direct the acoustic waves primarily in the direction of the fluid. In another or further embodiment, the sound waves may also be directed along the wall of the container. For example, the transducer can be configured to induce guided waves in the wall of the vessel, which then refract into the liquid and produce an acoustic (standing) wave field (in liquid: compressional waves). The acoustic compression wavefield then induces mixing of the liquids. Combinations are also contemplated, for example, some transducers configured to produce waves directly into the fluid, and other (or the same) transducers configured to produce guided waves in the vessel wall.

In some embodiments (not shown), the direction of the sound waves W1, W2 entering the fluid (sound flow direction) may be determined by combining individual waves generated by multiple acoustic transducers. For example, a phased array of transducers may be used, wherein the direction of the acoustic flow or combined wave may be determined by the relative phase of the individual waves of the respective transducers forming the array. In one embodiment, the vessel wall 11 may be lined with an array of transducers and the direction of flow switched by adjusting the relative phases of the actuating transducers.

In some embodiments, the mixing vessel 10 has a circular shape and the transducers are arranged to induce a circular flow along the vessel wall 11. For example, the mixing vessel 10 may have a cylindrical shape, e.g., as shown in fig. 2A and 2B; or have a ring shape, for example, as shown in fig. 4A and 4B. It is also contemplated to have an elliptical shape, for example, as shown in fig. 5A. Advantageously, using a circular (elliptical) shape mixing vessel 10 may more easily create a flow therethrough while minimizing stagnant areas (where there is less mixing). Alternatively, other shapes of the mixing container may also be used, such as rectangular as shown in fig. 4A, 4B, 6A, 6B; or for example, a polygon as shown in fig. 5B. The corners of the shape may help to create local vortices, which may also promote mixing.

Fig. 2A and 2B illustrate embodiments in which one or more

acoustic transducers

21, 22 are configured to induce a helical flow pattern F1, F2 in the mixing vessel 10. For example, the helical flow pattern may include a generally rotational flow component and a generally longitudinal flow component transverse to the rotation. In some embodiments, for example, as shown, the mixing vessel 10 has a cylindrical shape to direct a helical flow. For example, a group of one or more first acoustic transducers 21 is configured to induce a clockwise spiral flow, while a group of one or more second acoustic transducers 22 is configured to induce a counter-clockwise spiral flow. Advantageously, the helical flow may be guided by a cylindrical vessel wall 11. In some embodiments, the one or more transducers may be arranged to return fluid through the middle of the vessel.

Fig. 3A and 3B show flow patterns having opposite flow directions. In some embodiments, for example, as shown, the first transducer 21a is configured to direct the sound waves W1a of the first transducer along the first acoustic axis A1a in a first direction V1a, while the second transducer 21b disposed on the second wall 11b of the container is configured to direct the sound waves W1b of the second transducer along the second acoustic axis A1b (simultaneously) in a second direction V1 b. In one embodiment, the first direction V1a is opposite the second direction V1 b. In another or further embodiment, the first acoustic axis A1a is offset relative to the second acoustic axis A1 b. Advantageously, the configuration of opposing non-paraxial or shear flows (e.g., produced by vortices as shown) may provide improved mixing. This may improve mixing, for example. The opposite paraxial flow pattern is also contemplated, which may cause substantially turbulent mixing between the transducers. In some embodiments, for example, as shown, the first transducer 21a is disposed on a first wall 11a of the mixing vessel 10 and the second transducer 21b is disposed on an opposing second wall of the mixing vessel 10.

In some embodiments, for example, as shown or otherwise, one or more of the

transducers

21, 22 is configured to measure a respective flow pattern F1, F2. For example, some of the transducers may be used to measure flow velocity and/or flow direction. For example, the acoustic wave W1a may be generated by the first transducer 21a and measured by the second transducer 22b, the second transducer 22b being arranged in a path of the acoustic wave W1a, e.g., intersecting the acoustic axis. In one embodiment, the one or more transducers are configured to measure flow velocity by doppler shift. For example, a continuous wave transmitted by a first transducer may be received by a second transducer, wherein the frequency measured by the second transducer is doppler shifted with respect to the actuation of the first transducer according to the direction and/or velocity of flow between the first and second transducers. In another or further embodiment, the one or more transducers are configured to measure flow velocity by measuring time of arrival. For example, a pulsed wave is transmitted by a first transducer and may be received by a second transducer, wherein the measured time between transmission and reception may depend on the direction and/or speed of flow between the first transducer and the second transducer (downstream arrival being faster than upstream arrival).

In some embodiments, the actuation of the one or more transducers is controlled based on the flow measurements. For example, at least some actuators that are not used to generate flow may be used to measure flow. In one embodiment, the controller [ not shown here ] is configured to control the one or more

acoustic transducers

21, 22 to automatically switch between the generation of different sound waves W1, W2 based on the measurement results. For example, the flow may be switched when it is determined that laminar flow has been formed. Generally, in laminar flow, the flow direction and/or flow velocity may be substantially constant. In another or further embodiment, the controller is configured to control the one or more

acoustic transducers

21, 22 to automatically adjust one or more frequencies or intensities based on the measurement results to maintain the liquid velocity below a predetermined threshold. This may prevent damage to some liquids from excessive shear, for example.

Fig. 4A and 4B show a mixing vessel 10 in the shape of a ring or torus. In some embodiments, for example, as shown, the set of

transducers

21a, 21b is configured to induce opposite flow in the vessel, e.g., similar to that explained with reference to the previous figures. In other or further embodiments (not shown), it is also contemplated to induce a helical flow in the annular vessel. Advantageously, this enables a continuous helix to be induced around the channel formed by the container.

Fig. 5A and 5B show sound waves oriented at an angle relative to the opposing walls. In one embodiment, for example, as shown, the acoustic transducer 21 is arranged on the first wall 11a of the mixing vessel 10 and is configured to direct a sound wave W1 of the acoustic transducer along An acoustic axis a1 (central or main direction) in a direction V1 to impinge on the opposite (inner) second wall 11b of the mixing vessel 10 at An impingement angle β between the acoustic axis a1 and a normal An of the opposite second wall 11b, wherein the impingement angle β is greater than twenty degrees (plane angle), preferably greater than thirty degrees or even greater than forty degrees, for example between forty-five and seventy degrees. Advantageously, directing the flow direction V1 at an angle relative to the opposing wall may cause the flow to bounce off the wall and/or be directed along the wall. For example, a circulating flow may be formed that mixes the fluids. In a preferred embodiment, for example, as shown, there may be a second transducer 22 configured to induce an opposite flow pattern (not shown).

In some embodiments, for example, as shown in fig. 5A, the mixing vessel 10 may be circular, or in this case cylindrical. Advantageously, the transducer may be placed off-center (relative to the centerline of the ellipse) to strike the opposing wall at an angle. At the same time, the circular inner wall may enable easier formation of the circulating flow. In other or further embodiments, for example, as shown in fig. 5B, the mixing vessel 10 may have a polygonal shape, e.g., square, pentagonal, hexagonal, etc. Further, in this configuration, the acoustically induced flow may be directed by one or more

acoustic transducers

21, 22 to angularly impinge the opposing wall, thereby inducing a flow pattern along the wall. Advantageously, the vortices may be formed especially at the corners of the polygonal shape.

Fig. 6A and 6B show

acoustic transducers

21, 22 configured to direct their respective acoustic waves W1, W2 at a liquid/gas interface (L/G). Preferably, the wave hits the interface from the direction of the liquid (e.g. from below). Advantageously, waves passing through interfaces with different acoustic impedances can cause the formation of additional flows by radiation forces.

Without being bound by theory, acoustic radiation force may be understood as a nonlinear phenomenon of ultrasound propagation. Generally, the acoustic radiation force acts on an object or boundary having a difference in acoustic impedance compared to the original medium through which the acoustic wave propagates. If radiation forces act on a free boundary (i.e. a liquid-gas interface) on which the combined liquid jet (due to the acoustic flow) impinges, the liquid interface may start to vibrate, which may cause induced liquid flow. Radiation forces acting on liquid-solid boundaries (e.g., solid walls of solidity) do not generally cause additional liquid flow. However, if the compressible particles/bubbles are dispersed in the liquid medium (thus causing a difference in acoustic impedance at the location of the particles/bubbles), the particles/bubbles may start to move due to the radiation force. The particles/bubbles in turn move the liquid aside, causing the liquid to move. This is similar to the movement of liquid caused by the absorption of sound (acoustic streaming) in the liquid.

In some embodiments (not shown), the respective acoustic axes are oriented at an angle (e.g., greater than thirty degrees) relative to a normal to the interface to induce flow along the interface surface, similar to that explained in the previous figures. For example, in the illustrated embodiment, wedge elements may be disposed between one or more

acoustic transducers

21, 22 and the vessel wall 11 to guide waves; or the walls of the bottom may be inclined.

Fig. 7A shows the pressure distribution intensity "I" corresponding to one transducer 21. As shown, the sound waves "W" may be primarily directed along one acoustic axis "a" to induce a corresponding flow direction "V". Generally, the acoustic field is more directional when the wavelength of the acoustic wave is smaller than one or more dimensions of the transducer on the wall. In the case of a wave field generated by a transducer with a large opening angle (e.g. if the wavelength is large compared to one or more dimensions of the transducer, the wave field is generated), guided waves can be generated in the vessel wall. In some embodiments, the frequency of the transducer may be switched between a first mode in which the wavelength of the acoustic wave (e.g., in the vessel wall and/or fluid) is greater than the extent of the transducer (e.g., the diameter along the wall); in the second mode, the wavelength is less than the range of the transducer. This may therefore induce different modes/wave directions. Of course, other frequency variations are also contemplated to switch between different modes. In one embodiment, frequency scanning is applied, for example, for an unfocused transducer, the low frequency generated acoustic field is different from the high frequency generated acoustic field. There may also be a combination of low frequency components and high frequency components.

Fig. 7B shows interference between sound waves of different (e.g., adjacent)

transducers

21, 22. As shown, interference of different waves may cause constructive and/or destructive interference. In some embodiments, the distance between

adjacent transducers

21, 22 may be less than the wavelength λ of the acoustic wave (e.g., in the fluid). In some embodiments, constructive interference between the sound waves of the

different transducers

21, 22 may cause one or more secondary beams (grating lobes) along the secondary axis a', where the pressure variations or sound flow are relatively high.

Without being bound by theory, it is observed that the direction of the secondary axis is wavelength dependent, e.g. constructive interference occurs at positions in the fluid where the distance relative to the different transducers is an integer multiple of the wavelength. This may be similar to an (optical) grating. It will be appreciated that the direction of the secondary axis may be controlled, for example by controlling the frequency of the transducer. In some embodiments, the frequency of the transducers may be switched between a first mode in which the wavelength of the acoustic waves (e.g., in the vessel wall and/or fluid) is greater than the (center) distance D between the transducers (e.g., along the wall); in the second mode, the wavelength is less than the distance. It is also conceivable to switch between three different frequencies. For example, in a first mode where the frequency is relatively low, there may be no grating lobes; at higher frequencies, grating lobes can be generated; at higher frequencies, the grating lobes move towards the main beam.

Furthermore, it is contemplated that other variations may be combined or separated from the frequency variation. In one embodiment, the amplitude modulation of the wave field may be produced by a single transducer or multiple transducers. In another or further embodiment, the length of the sine wave pulses generated by the one or more transducers may vary over time. In some embodiments, the shape or size of the different transducers may differ between different modes. In one embodiment, a first transducer actuated in a first mode has a first diameter and a second transducer actuated in a second mode has a second diameter, which may be smaller or larger than the first diameter. In another or further embodiment, the transducer comprises an annular array, for example comprising (concentric rings) of different sizes or diameters. Transducers of different sizes may actuate at the same or different frequencies. For example, switching between transducers may cause a change in the shape of the acoustic field, for example, because the source aperture changes. Furthermore, the efficiency of inducing acoustic streaming can vary (e.g., by the square of the diameter dependence discussed by the following equation). This may provide a further effect when the frequencies are different (frequency dependence is also discussed below). The combination of high and low frequency components may also be used to optimize the velocity field of the induced fluid. Of course, different options may be combined.

Acoustic streaming of a liquid is induced by absorption of acoustic waves during their propagation through the liquid. Thus, depending on the shape of the field and the properties of the medium (liquid/gas), acoustic streaming can occur in all acoustic radiation fields. Without being bound by theory, acoustic flow may generally be related to acoustic attenuation in a fluid. The inventors have found that the liquid velocity induced by acoustic streaming can be approximated by the following proportional relationship:

wherein "V" is the induced (peak) liquid velocity; "P" is the acoustic pressure in the fluid (e.g., P²Can be compared with the sound intensity I at the surface of the transducer₀Proportional); "a" is the radius (or diameter) of the transducer; "c₀"is the acoustic velocity in the fluid; mu₀"is the viscosity of the fluid; ' d_c"is the duty cycle of the transducer; "f" is the frequency of the acoustic wave; "n" is a number between one and two.

In some embodiments, the acoustic pressure or intensity at the transducer surface may be controlled to provide a desired liquid velocity. In other or further embodiments, it may be desirable to prevent damage to a liquid (e.g., milk) by maintaining a relatively low peak pressure in the liquid (e.g., less than one mega pascal, preferably less than five hundred kilopascals, more preferably less than three hundred kilopascals, e.g., between one kilopascal and two hundred kilopascals). This may also depend on, for example, frequency.

In some embodiments, the frequency of the transducer is controlled to provide a desired liquid velocity. For example, the frequency for mixing the liquids is chosen to be between 0.1MHz and 100MHz, preferably between 0.5MHz and 5MHz, more preferably between 0.8MHz and 3 MHz. In some embodiments, the transducer is configured to operate in a resonant mode to increase power efficiency.

In some embodiments, one or more, preferably all, of the acoustic transducers may be relatively large, for example, greater than one centimeter in diameter (along the vessel wall), greater than two centimeters, greater than five centimeters, or even greater than ten centimeters. As the relationship above shows, increasing the size of the transducer can more effectively achieve the desired fluid velocity.

In some embodiments, it is desirable to maintain a relatively low peak liquid velocity, e.g., less than one meter per second, less than one-half meter per second, less than 0.3m/s, or even less. For example, in some liquids (e.g., milk), it may be desirable to maintain a relatively low peak liquid velocity (e.g., between 0.01m/s and 0.3m/s, preferably less than 0.2m/s) to prevent damage from shearing.

To prevent high peak speeds while still providing sufficient mixing, for example, a relatively large number of low power transducers may be used. In some embodiments, in the mixing vessel, at least one transducer may be used per two hundred liters of liquid mixed, per one hundred liters of liquid mixed, per fifty liters of liquid mixed, per ten liters of liquid mixed, or even per one liter of liquid mixed. In other or further embodiments, the supply power to each transducer may be less than one hundred watts, less than fifty watts, less than twenty watts, or even less than ten watts, e.g., the supply power to each transducer may be between one and five watts. For example, mixing in a 4000 liter milk tank may use forty transducers with a total power of about 100 watts.

In a preferred application (e.g., maintaining a storage container with fluid in a mixed state), the mixing container has a relatively large volume. For example, the container is configured to hold a volume of fluid in excess of one liter, in excess of one hundred liters, or even in excess of one kiloliter (cubic meter) (e.g., between four kiloliters and ten thousand liters, or more). For example, the system may be applied in a container for storing and/or transporting milk, for example in a container behind a truck. To mix relatively large volumes of fluid, or to keep the fluids mixed, many arrangements of acoustic transducers may be used. For example, more than ten, more than fifty, or even more than a hundred acoustic transducers may be used.

The power required to mix the fluids (or keep the fluids mixed) may vary depending on the configuration of the transducers, the shape of the mixing vessel, and the type of fluid. For example, with the optimization described herein, it was found that the power required to mix a tank of four kiloliters of milk is approximately between one hundred watts and one thousand watts. Depending on efficiency, a significant portion of this power may be dissipated in the form of heat in the mixed fluid. For example, dissipating 1kW of power in 4000kg of liquid with a heat capacity of 4kJ/kg K will cause a negligible temperature increase ((1kW/4000kg)/(4kJ/kg K) ═ 0.000062K/s) in about five minutes.

In some embodiments, it is preferable to keep the energy dissipated in the fluid relatively low upon mixing. In a preferred embodiment, the configuration is adapted to dissipate less than ten watts per liter, less than one watt per liter, less than half a watt per liter, or even less than one tenth watt per liter (0.1W/l). In other or further embodiments, measures may be taken to prevent heating of the fluid by acoustic mixing. In one embodiment, the apparatus 100 includes an active cooler to at least partially or even completely counteract the fluid heating caused by the acoustic transducer. For example, the cooling capacity of an active cooler is at least equal to the heat dissipation of the sound waves in the fluid. For example, the active cooler may be controlled based on a temperature measurement of the fluid. In some embodiments, the cooling may be switched based on actuation of the acoustic transducer. In one embodiment, the one or more acoustic transducers are configured to induce, among other things, fluid flow along the actively cooled surface.

It will be appreciated that the teachings of the present invention for non-contact mixing are particularly applicable to applications where contamination prevention is important when mixing fluids (or keeping fluids mixed), for example, in the food industry, pharmaceutical industry, or general chemical industry. In some embodiments, the fluids being mixed have a relatively large viscosity (compared to water), for example, a viscosity in excess of two centipoise (millipascal seconds). For example, milk typically has a viscosity of three centipoise (at room temperature). In one embodiment, the fluid being mixed is milk, wherein the configuration is controlled to maintain a peak liquid velocity below thirty centimeters per second and a peak sound pressure below one megapascal.

For purposes of clarity and conciseness of description, features are described herein as part of the same or separate embodiments, however, it is to be understood that the scope of the invention may include embodiments having combinations of all or some of the features described. For example, although embodiments for switching different flow patterns are shown, alternative ways may be envisioned by those skilled in the art having the benefit of this disclosure to achieve similar functions and results. For example, different configurations may be combined or separated into one or more alternative components. The different elements of the embodiments discussed and illustrated provide advantages, such as mixing of easily damaged fluids. Of course, it is to be understood that any one of the above-described embodiments or processes may be combined with one or more other embodiments or processes to provide further improvements in finding and matching designs and advantages. It will be appreciated that the present disclosure provides particular advantages to the food industry and may be applied generally to any application where fluids (e.g., liquids or gases) are to be mixed or are to remain mixed.

In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit the scope of the claims; multiple "devices" may be represented by the same or different items or implement structures or functions; any disclosed devices or components may be combined together or separated into other components unless specifically stated otherwise. This may indicate a synergistic advantage achieved by a combination of their respective features when one claim is dependent on another. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. Embodiments of the invention may thus include all working combinations of claims, wherein each claim may in principle refer to any preceding claim unless explicitly excluded by context.

Claims

1. An apparatus (100) for mixing a fluid (F), the apparatus comprising:

-a mixing container (10) comprising a container wall (11) for containing the fluid (F);

-at least one acoustic transducer (21, 22) arranged on the vessel wall (11) and configured to generate respective sound waves (W1, W2) directed into the fluid (F) along a respective acoustic axis (a1, a2) for inducing a respective flow pattern (F1, F2) in the fluid (F) by acoustic flow, wherein the acoustic axis (a1, a2) is at a respective angle (α 1, α 2) greater than thirty degrees with respect to a normal (An) of the vessel wall (11); and

-a controller (15) configured to control the at least one acoustic transducer (21, 22) to automatically switch between the generation of different sound waves (W1, W2) to cause switching between different flow patterns (F1, F2).

2. The apparatus of claim 1, wherein the one or more acoustic transducers (21, 22) are configured to induce a first flow pattern (F1) by generating a first set of sound waves (W1) over a first time period (T1), and then automatically switch to induce a second, different flow pattern (F2) by generating a second, different set of sound waves (W2) over a second time period (T2).

3. The apparatus of claim 2, wherein a first subset of at least one transducer (21) is configured to induce the first flow pattern (F1) and a different second subset of at least one other transducer (22) is configured to induce the second flow pattern (F2).

4. Apparatus according to claim 2 or 3, wherein the respective angles (a1, a2) of the sound waves (W1, W2) are oppositely directed along the circumference of the vessel wall (11) between different flow patterns (F1, F2) for having the first flow pattern (F1) with a first flow direction (V1) tangential to the vessel wall at one location in the mixing vessel (10) and the second flow pattern (F2) with an opposite second flow direction (V2) tangential to the vessel wall at the same location in the mixing vessel (10).

5. The apparatus of any one of the preceding claims, wherein the respective angle (a1, a2) is determined by at least one wedge element (11w) arranged between the acoustic transducer (21, 22) and the vessel wall (11); the container wall (11) comprises or forms a wedge-shaped surface against which the at least one acoustic transducer (21, 22) is mounted; and the at least one acoustic transducer (21, 22) is at least partially buried within the vessel wall (11) at an angle relative to a normal to an inner surface of the vessel wall (11).

6. The apparatus of any preceding claim, wherein the mixing vessel (10) has a circular shape and the transducers are arranged to induce a circulating flow along the vessel wall (11).

7. The apparatus according to any one of the preceding claims, wherein the mixing vessel (10) has a cylindrical or annular shape and one or more acoustic transducers (21, 22) are configured to induce a helical flow pattern in the mixing vessel.

8. Apparatus according to any one of the preceding claims, wherein a first transducer (21a) is configured to direct its sound waves (W1a) along a first acoustic axis (A1a) in a first direction (V1a), while a second transducer (21b) arranged on a second wall (11b) of the container is configured to direct its sound waves (W1b) along a second acoustic axis (A1b) in a second direction (V1b), wherein the first direction (V1a) is opposite to the second direction (V1 b); and the first acoustic axis (A1a) is offset relative to the second acoustic axis (A1 b).

9. The apparatus of any preceding claim, wherein An acoustic transducer (21) is arranged on the first wall (11a) of the mixing vessel (10) and is configured to direct sound waves (W1) of the acoustic transducer in a direction (V1) along An acoustic axis (a1) to impinge on the opposing second wall (11b) of the mixing vessel (10) at An impingement angle (β) between the acoustic axis (a1) and a normal (An) to the opposing second wall (11b), wherein the impingement angle (β) is greater than thirty degrees.

10. The apparatus of any preceding claim, wherein one or more acoustic transducers (21, 22) are configured to direct their respective sound waves (W1, W2) at a liquid/gas (L/G) interface.

11. The apparatus of any one of the preceding claims, wherein one or more transducers (21, 22) are configured to measure respective flow patterns (F1, F2).

12. The device according to claim 11, wherein the controller (15) is configured to control one or more acoustic transducers (21, 22) to automatically switch between the generation of different sound waves (W1, W2) based on the measurement results.

13. The apparatus of claim 11 or 12, wherein the controller (15) is configured to control the one or more acoustic transducers (21, 22) to automatically adjust one or more frequencies or intensities based on the measurement results to maintain the liquid velocity below a predetermined threshold.

14. A method for mixing a fluid (F), the method comprising:

-containing said fluid (F) in a mixing container (10);

-generating respective sound waves (W1, W2) directed into the fluid (F) along respective acoustic axes (a1, a2) for inducing respective flow patterns (F1, F2) in the fluid (F) by acoustic flow, wherein the acoustic axes (a1, a2) are at respective angles (α 1, α 2) greater than thirty degrees with respect to a normal (An) of the vessel wall (11); and

-automatically switching between the generation of different sound waves (W1, W2) to cause switching between different flow patterns (F1, F2).

15. The method of claim 14, wherein the fluid is milk and the peak liquid velocity is maintained below thirty centimeters per second and the peak sound pressure is maintained below one megapascal.